HTTP Working Group Koen Holtman, TUE
Internet-Draft Andrew Mutz, Hewlett-Packard
Expires: January 26, 1998 July 26, 1997
Transparent Content Negotiation in HTTP
draft-ietf-http-negotiation-03.txt
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ABSTRACT
HTTP allows web site authors to put multiple versions of the
same information under a single URL. Transparent content
negotiation is an extensible negotiation mechanism, layered on
top of HTTP, for automatically selecting the best version when
the URL is accessed. This enables the smooth deployment of
new web data formats and markup tags.
OVERVIEW OF THE TRANSPARENT CONTENT NEGOTIATION DOCUMENT SET
An up-to-date overview of documents related to transparent content
negotiation is maintained on the web page
<URL:http://gewis.win.tue.nl/~koen/conneg/>.
The transparent content negotiation document set currently consists
of two series of internet drafts.
1. draft-ietf-http-negotiation-XX.txt (this document)
`Transparent Content Negotiation in HTTP'
Defines the core mechanism. Experimental track.
2. draft-ietf-http-rvsa-v10-XX.txt
`HTTP Remote Variant Selection Algorithm -- RVSA/1.0'
Defines the remote variant selection algorithm version 1.0.
Experimental track.
Two related series of drafts are
3. draft-ietf-http-feature-reg-XX.txt
`Feature Tag Registration Procedures'
Defines feature tag registration. Best Current Practice
track.
4. draft-ietf-http-feature-scenarios-XX.txt
`Feature Tag Scenarios'
Discusses feature tag scenarios. Informational track.
An additional document about `the core feature set', which may
later become an informational RFC, may also appear. Currently,
there are two internet drafts which discuss parts of what could be
a core feature set: draft-mutz-http-attributes-XX.txt and
draft-goland-http-headers-XX.txt
Older versions of the text in documents 1 and 2 may be found in the
draft-holtman-http-negotiation-XX.txt series of internet drafts.
TABLE OF CONTENTS
1 Introduction
1.1 Background
1.2 Revision history
2 Terminology
2.1 Terms from HTTP/1.1
2.2 New terms
3 Notation
4 Overview
4.1 Content negotiation
4.2 HTTP/1.0 style negotiation scheme
4.3 Transparent content negotiation scheme
4.4 Optimizing the negotiation process
4.5 Downwards compatibility with non-negotiating user agents
4.6 Retrieving a variant by hand
4.7 Dimensions of negotiation
4.8 Feature negotiation
4.9 Length of variant lists
4.10 Relation with other negotiation schemes
5 Variant descriptions
5.1 Syntax
5.2 URI
5.3 Source-quality
5.4 Type, charset, language, and length
5.5 Features
5.6 Description
5.7 Extension-attribute
6 Feature negotiation
6.1 Feature tags
6.1.1 Feature tag values
6.2 Feature sets
6.3 Feature predicates
6.4 Features attribute
7 Remote variant selection algorithms
7.1 Version numbers
8 Content negotiation status codes and headers
8.1 506 Variant Also Negotiates
8.2 Accept-Features
8.3 Alternates
8.4 Negotiate
8.5 TCN
8.6 Variant-Vary
9 Cache validators
9.1 Variant list validators
9.2 Structured entity tags
9.3 Assigning entity tags to variants
10 Content negotiation responses
10.1 List response
10.2 Choice response
10.3 Adhoc response
10.4 Reusing the Alternates header
10.5 Extracting a normal response from a choice response
10.6 Elaborate Vary headers
10.6.1 Construction of an elaborate Vary header
10.6.2 Caching of an elaborate Vary header
10.7 Adding an Expires header to ensure HTTP/1.0 compatibility
10.8 Negotiation on content encoding
11 User agent support for transparent negotiation
11.1 Handling of responses
11.2 Presentation of a transparently negotiated resource
12 Origin server support for transparent negotiation
12.1 Requirements
12.2 Negotiation on transactions other than GET and HEAD
13 Proxy support for transparent negotiation
14 Security and privacy considerations
14.1 Accept- headers revealing information of a private nature
14.2 Spoofing of responses from variant resources
14.3 Security holes revealed by negotiation
15 Acknowledgments
16 References
17 Authors' addresses
18 Appendix: Example of a local variant selection algorithm
18.1 Computing overall quality values
18.2 Determining the result
18.3 Ranking dimensions
19 Appendix: feature negotiation examples
19.1 Use of feature tags
19.2 Use of numeric feature tags
19.3 Feature tag design
20 Appendix: origin server implementation considerations
20.1 Implementation with a CGI script
20.2 Direct support by HTTP servers
20.3 Web publishing tools
21 Appendix: Example of choice response construction
1 Introduction
HTTP allows web site authors to put multiple versions of the same
information under a single URI. Each of these versions is called a
`variant'. Transparent content negotiation is an extensible
negotiation mechanism for automatically and efficiently retrieving
the best variant when a GET or HEAD request is made. This enables
the smooth deployment of new web data formats and markup tags.
This specification defines transparent content negotiation as an
extension on top of the HTTP/1.1 protocol [1]. However, use of
this extension does not require use of HTTP/1.1: transparent
content negotiation can also be done if some or all of the parties
are HTTP/1.0 [3] systems.
Transparent content negotiation is called `transparent' because it
makes all variants which exist inside the origin server visible to
outside parties.
Note: Though this specification is limited to negotiation on
HTTP transactions, elements of this specification could also be
used in other contexts. For example, feature predicates could
be used in conditional HTML, and variant descriptions could be
used in multipart mail messages. Such use in other contexts is
encouraged.
1.1 Background
The addition of content negotiation to the web infrastructure has
been considered important since the early days of the web. Among
the expected benefits of a sufficiently powerful system for content
negotiation are
* smooth deployment of new data formats and markup tags will
allow graceful evolution of the web
* eliminating the need to choose between a `state of the art
multimedia homepage' and one which can be viewed by all web
users
* enabling good service to a wider range of browsing
platforms (from low-end PDA's to high-end VR setups)
* eliminating error-prone and cache-unfriendly
User-Agent based negotiation
* enabling construction of sites without `click here for the X
version' links
* internationalization, and the ability to offer multi-lingual
content without a bias towards one language.
1.2 Revision history
There are no major changes in this version. The Content-Features
header has been deleted, and some new text on security
considerations was added. Some existing text has been improved.
2 Terminology
The words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in
this document are to be interpreted as described in RFC 2119 [6].
This specification used the term `header' as an abbreviation for
for `header field in a request or response message'.
2.1 Terms from HTTP/1.1
This specification mostly uses the terminology of the HTTP/1.1
specification [1]. The definitions below were reproduced from [1].
request
An HTTP request message.
response
An HTTP response message.
resource
A network data object or service that can be identified by a URI.
Resources may be available in multiple representations
(e.g. multiple languages, data formats, size, resolutions) or
vary in other ways.
content negotiation
The mechanism for selecting the appropriate representation when
servicing a request.
client
A program that establishes connections for the purpose of sending
requests.
user agent
The client which initiates a request. These are often browsers,
editors, spiders (web-traversing robots), or other end user
tools.
server
An application program that accepts connections in order to
service requests by sending back responses. Any given program may
be capable of being both a client and a server; our use of these
terms refers only to the role being performed by the program for
a particular connection, rather than to the program's
capabilities in general. Likewise, any server may act as an
origin server, proxy, gateway, or tunnel, switching behavior
based on the nature of each request.
origin server
The server on which a given resource resides or is to be created.
proxy
An intermediary program which acts as both a server and a client
for the purpose of making requests on behalf of other
clients. Requests are serviced internally or by passing them on,
with possible translation, to other servers. A proxy must
implement both the client and server requirements of this
specification.
age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
fresh
A response is fresh if its age has not yet exceeded its freshness
lifetime.
2.2 New terms
transparently negotiable resource
A resource, identified by a single URI, which has multiple
representations (variants) associated with it. When servicing a
request on its URI, it allows selection of the best
representation using the transparent content negotiation
mechanism. A transparently negotiable resource always has a
variant list bound to it, which can be represented as an
Alternates header (defined in section 8.3).
variant list
A list containing variant descriptions, which can be bound to a
transparently negotiable resource.
variant description
A machine-readable description of a variant resource, usually
found in a variant list. A variant description contains the
variant resource URI and various attributes which describe
properties of the variant. Variant descriptions are defined in
section 5.
variant resource
A resource from which a variant of a negotiable resource can be
retrieved with a normal HTTP/1.x GET request, i.e. a GET request
which does not use transparent content negotiation.
neighboring variant
A variant resource is called a neighboring variant resource of
some transparently negotiable HTTP resource if the variant
resource has a HTTP URL, and if the absolute URL of the variant
resource up to its last slash equals the absolute URL of the
negotiable resource up to its last slash, where equality is
determined with the URI comparison rules in section 3.2.3 of [1].
The property of being a neighboring variant is important because
of security considerations (section 14.2). Not all variants of a
negotiable resource need to be neighboring variants. However,
access to neighboring variants can be more highly optimized by
the use of remote variant selection algorithms (section 7) and
choice responses (section 10.2).
remote variant selection algorithm
A standardized algorithm by which a server can sometimes choose a
best variant on behalf of a negotiating user agent. The
algorithm typically computes whether the Accept- headers in the
request contain sufficient information to allow a choice, and if
so, which variant is the best variant. The use of a remote
algorithm can speed up the negotiation process.
list response
A list response returns the variant list of the negotiable
resource, but no variant data. It can be generated when the
server does not want to, or is not allowed to, return a
particular best variant for the request. List responses are
defined in section 10.1.
choice response
A choice response returns a representation of the best variant
for the request, and may also return the variant list of the
negotiable resource. It can be generated when the server has
sufficient information to be able to choose the best variant on
behalf the user agent, but may only be generated if this best
variant is a neighboring variant. Choice responses are defined
in section 10.2.
adhoc response
An adhoc response can be sent by an origin server as an extreme
measure, to achieve compatibility with a non-negotiating or buggy
client if this compatibility cannot be achieved by sending a list
or choice response. There are very little requirements on the
contents of an adhoc response. Adhoc responses are defined in
section 10.3.
Accept- headers
The request headers: Accept, Accept-Charset, Accept-Language, and
Accept-Features.
supports transparent content negotiation
From the viewpoint of an origin server or proxy, a user agent
supports transparent content negotiation if and only if it sends
a Negotiate header (section 8.4) which indicates such support.
server-side override
If a request on a transparently negotiated resource is made by a
client which supports transparent content negotiation, an origin
server is said to perform a server-side override if the server
ignores the directives in the Negotiate request header, and
instead uses a custom algorithm to choose an appropriate
response. A server-side override can sometimes be used to work
around known client bugs. It could also be used by protocol
extensions on top of transparent content negotiation.
3 Notation
The version of BNF used in this document is taken from [1], and
many of the nonterminals used are defined in [1].
One new BNF construct is added:
1%rule
stands for one or more instances of "rule", separated by
whitespace:
1%rule = rule *( 1*LWS rule )
This specification also introduces
number = 1*DIGIT
short-float = 1*3DIGIT [ "." 0*3DIGIT ]
This specification uses the same conventions as in [1] (see section
1.2 of [1]) for defining the significance of each particular
requirement.
4 Overview
This section gives an overview of transparent content negotiation.
It starts with a more general discussion of negotiation as provided
by HTTP.
4.1 Content negotiation
HTTP/1.1 allows web site authors to put multiple versions of the
same information under a single resource URI. Each of these
versions is called a `variant'. For example, a resource
http://x.org/paper could bind to three different variants of a
paper:
1. HTML, English
2. HTML, French
3. Postscript, English
Content negotiation is the process by which the best variant is
selected if the resource is accessed. The selection is done by
matching the properties of the available variants to the
capabilities of the user agent and the preferences of the user.
It has always been possible under HTTP to have multiple
representations available for one resource, and to return the most
appropriate representation for each subsequent request. However,
HTTP/1.1 is the first version of HTTP which has provisions for
doing this in a cache-friendly way. These provisions include the
Vary response header, entity tags, and the If-None-Match request
header.
4.2 HTTP/1.0 style negotiation scheme
The HTTP/1.0 protocol elements allow for a negotiation scheme as
follows:
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
| GET http://x.org/paper
| Accept- headers
choose
|
---------------------------------- >
Best variant
When the resource is accessed, the user agent sends (along with its
request) various Accept- headers which express the user agent
capabilities and the user preferences. Then the origin server uses
these Accept- headers to choose the best variant, which is returned
in the response.
The biggest problem with this scheme is that it does not scale
well. For all but the most minimal user agents, Accept- headers
expressing all capabilities and preferences would be very large,
and sending them in every request would be hugely inefficient, in
particular because only a small fraction of the resources on the
web have multiple variants.
4.3 Transparent content negotiation scheme
The transparent content negotiation scheme eliminates the need to
send huge Accept- headers, and nevertheless allows for a selection
process that always yields either the best variant, or an error
message indicating that user agent is not capable of displaying any
of the available variants.
Under the transparent content negotiation scheme, the server sends
a list with the available variants and their properties to the user
agent. An example of a list with three variants is
{"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript} {language en}}
The syntax and semantics of the variant descriptions in this list
are covered in section 5. When the list is received, the user
agent can choose the best variant and retrieve it. Graphically,
the communication can be represented as follows:
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
| GET http://x.org/paper
|
----------------------------------- > [list response]
return of list |
choose
|
< ----------------------------------
| GET http://x.org/paper.1
|
---------------------------------- > [normal response]
return of paper.1
The first response returning the list of variants is called a `list
response'. The second response is a normal HTTP response: it does
not contain special content negotiation related information. Only
the user agent needs to know that the second request actually
retrieves a variant. For the other parties in the communication,
the second transaction is indistinguishable from a normal HTTP
transaction.
With this scheme, information about capabilities and preferences is
only used by the user agent itself. Therefore, sending such
information in large Accept- headers is unnecessary. Accept-
headers do have a limited use in transparent content negotiation
however; the sending of small Accept- headers can often speed up the
negotiation process. This is covered in section 4.4.
List responses are covered in section 10.1. As an example, the
list response in the above picture could be:
HTTP/1.1 300 Multiple Choices
Date: Tue, 11 Jun 1996 20:02:21 GMT
TCN: list
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Vary: negotiate, accept, accept-language
ETag: "blah;1234"
Cache-control: max-age=86400
Content-Type: text/html
Content-Length: 227
<h2>Multiple Choices:</h2>
<ul>
<li><a href=paper.1>HTML, English version</a>
<li><a href=paper.2>HTML, French version</a>
<li><a href=paper.3>Postscript, English version</a>
</ul>
The Alternates header in the response contains the variant list.
The Vary header is included to ensure correct caching by plain
HTTP/1.1 caches (see section 10.6). The ETag header allows the
response to be revalidated by caches, the Cache-Control header
controls this revalidation. The HTML entity included in the
response allows the user to select the best variant by hand if
desired.
4.4 Optimizing the negotiation process
The basic transparent negotiation scheme involves two HTTP
transactions: one to retrieve the list, and a second one to retrieve
the chosen variant. There are however several ways to `cut corners'
in the data flow path of the basic scheme.
First, caching proxies can cache both variant lists and variants.
Such caching can reduce the communication overhead, as shown in the
following example:
Server _____ proxy _____ proxy __________ user
x.org cache cache agent
< --------------
| GET ../paper
|
has the list
in cache
|
------------- > [list response]
list |
|
choose
|
< --------------------------
| GET ../paper.1
|
has the variant
in cache
|
-------------------------- > [normal response]
return of paper.1
Second, the user agent can send small Accept- headers, which may
contain enough information to allow the server to choose the best
variant and return it directly.
Server _____ proxy _____ proxy _____ user
x.org cache cache agent
< ----------------------------------
| GET http://x.org/paper
| small Accept- headers
|
able to choose on
behalf of user agent
|
---------------------------------- > [choice response]
return of paper.1 and list
This choosing based on small Accept- headers is done with a `remote
variant selection algorithm'. Such an algorithm takes the variant
list and the Accept- headers as input. It then computes whether the
Accept- headers contain sufficient information to choose on behalf
of the user agent, and if so, which variant is the best variant.
If the best variant is a neighboring variant, it may be returned,
together with the variant list, in a choice response.
A server may only choose on behalf of a user agent supporting
transparent content negotiation if the user agent explicitly allows
the use of a particular remote variant selection algorithm in the
Negotiate request header. User agents with sophisticated internal
variant selection algorithms may want to disallow a remote choice,
or may want to allow it only when retrieving inline images. If the
local algorithm of the user agent is superior in only some
difficult areas of negotiation, it is possible to enable the remote
algorithm for the easy areas only. More information about the use
of a remote variant selection algorithm can be found in [5].
Choice responses are covered in section 10.2. For example, the
choice response in the above picture could be:
HTTP/1.1 200 OK
Date: Tue, 11 Jun 1996 20:05:31 GMT
TCN: choice
Content-Type: text/html
Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
Content-Length: 5327
Cache-control: max-age=604800
Content-Location: paper.html.en
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Etag: "gonkyyyy;1234"
Vary: negotiate, accept, accept-language
Expires: Thu, 01 Jan 1980 00:00:00 GMT
<title>A paper about ....
Finally, the above two kinds of optimization can be combined; a
caching proxy which has the list will sometimes be able to choose on
behalf of the user agent. This could lead to the following
communication pattern:
Server _____ proxy _____ proxy __________ user
x.org cache cache agent
< ---------------
| GET ../paper
| small Accept
|
able to choose
on behalf
|
< ----------
| GET ../paper.1
|
---------- > [normal response]
html.en |
---------------- > [choice response]
paper.1 and list
Note that this cutting of corners not only saves bandwidth, it also
eliminates delays due to packet round trip times, and reduces the
load on the origin server.
4.5 Downwards compatibility with non-negotiating user agents
To handle requests from user agents which do not support
transparent content negotiation, this specification allows the
origin server to revert to a HTTP/1.0 style negotiation scheme.
The specification of heuristics for such schemes is beyond the
scope of this document.
4.6 Retrieving a variant by hand
It is always possible for a user agent to retrieve the variant list
which is bound to a negotiable resource. The user agent can use
this list to make available a menu of all variants and their
characteristics to the user. Such a menu allows the user to
randomly browse other variants, and makes it possible to manually
correct any sub-optimal choice made by the automatic negotiation
process.
4.7 Dimensions of negotiation
Transparent content negotiation defines four dimensions of
negotiation:
1. Media type (MIME type)
2. Charset
3. Language
4. Features
The first three dimensions have traditionally been present in HTTP.
The fourth dimension is added by this specification. Additional
dimensions, beyond the four mentioned above, could be added by
future specifications.
Negotiation on the content encoding of a response (gzipped,
compressed, etc.) is left outside of the realm of transparent
negotiation. See section 10.8 for more information.
4.8 Feature negotiation
Feature negotiation intends to provide for all areas of negotiation
not covered by the type, charset, and language dimensions.
Examples are negotiation on
* HTML extensions
* Extensions of other media types
* Color capabilities of the user agent
* Screen size
* Output medium (screen, paper, ...)
* Preference for speed vs. preference for graphical detail
The feature negotiation framework (section 6) is the principal
means by which transparent negotiation offers extensibility; a new
dimension of negotiation (really a sub-dimension of the feature
dimension) can be added without the need for a new standards effort
by the simple registration of a `feature tag'. Feature tag
registration is discussed in [4].
4.9 Length of variant lists
As a general rule, variant lists should be short: it is expected
that a typical transparently negotiable resource will have 2 to 10
variants, depending on its purpose. Variant lists should be short
for a number of reasons:
1. The user must be able to pick a variant by hand to correct a
bad automatic choice, and this is more difficult with a long
variant list.
2. A large number of variants will decrease the efficiency of
internet proxy caches.
3. Long variant lists will make some transparently negotiated
responses longer.
In general, it is not desirable to create a transparently
negotiable resource with hundreds of variants in order to fine-tune
the graphical presentation of a resource. Any graphical
fine-tuning should be done, as much as possible, by using
constructs which act at the user agent side, for example
<center><img src=titlebanner.gif width=100%
alt="MegaBozo Corp"></center>
In order to promote user agent side fine tuning, which is more
scalable than fine tuning over the network, user agents which
implement a scripting language for content rendering are encouraged
to make the availability of this language visible for transparent
content negotiation, and to allow rendering scripts to access the
capabilities and preferences data used for content negotiation, as
far as privacy considerations permit this.
4.10 Relation with other negotiation schemes
The HTTP/1.x protocol suite allows for many different negotiation
mechanisms. Transparent content negotiation specializes in
scalable, interoperable negotiation of content representations at
the HTTP level. It is intended that transparent negotiation will
co-exist with other negotiation schemes, both open and proprietary,
which cover different application domains or work at different
points in the author-to-user chain. Ultimately, it will be up to
the resource author to decide which negotiation mechanism, or
combination of negotiation mechanisms, is most appropriate for the
task at hand.
This specification explicitly encourages future negotiation
mechanisms to re-use parts of this specification when appropriate.
With respect to re-use, two parts of this specification are
particularly important:
1. the syntax and semantics of variant descriptions (section 5-6)
2. the transport and caching protocol for negotiated data (section
8-10)
5 Variant descriptions
5.1 Syntax
A variant can be described in a machine-readable way with a variant
description.
variant-description =
"{" <"> URI <"> source-quality *variant-attribute"}"
source-quality = qvalue
variant-attribute = "{" "type" media-type "}"
| "{" "charset" charset "}"
| "{" "language" 1#language-tag "}"
| "{" "length" 1*DIGIT "}"
| "{" "features" feature-list "}"
| "{" "description" quoted-string "}"
| extension-attribute
extension-attribute = "{" extension-name extension-value "}"
extension-name = token
extension-value = *( token | quoted-string | LWS
| extension-specials )
extension-specials =
<any element of tspecials except <"> and "}">
The feature-list syntax is defined in section 6.4.
Examples are
{"paper.2" 0.7 {type text/html} {language fr}}
{"paper.5" 0.9 {type text/html} {features tables}}
{"paper.1" 0.001}
The various attributes which can be present in a variant
description are covered in the subsections below. Each attribute
may appear only once in a variant description.
5.2 URI
The URI attribute gives the URI of the resource from which the
variant can be retrieved with a GET request. It can be absolute or
relative to the Request-URI. The variant resource may vary (on the
Cookie request header, for example), but MUST NOT engage in
transparent content negotiation itself.
5.3 Source-quality
The source-quality attribute gives the quality of the variant, as a
representation of the negotiable resource, when this variant is
rendered with a perfect rendering engine on the best possible
output medium.
If the source-quality is less than 1, it often expresses a quality
degradation caused by a lossy conversion to a particular data
format. For example, a picture originally in JPEG form would have
a lower source quality when translated to the XBM format, and a
much lower source quality when translated to an ASCII-art variant.
Note however, that degradation is a function of the source; an
original piece of ASCII-art may degrade in quality if it is
captured in JPEG form.
The source-quality could also represent a level of quality caused
by skill of language translation, or ability of the used media type
to capture the intended artistic expression.
It is important that content providers do not assign very low
source quality values without good reason, as this would limit the
ability of users to influence the negotiation process with their
own preference settings. The following table SHOULD be used as a
guide when assigning source quality values:
1.000 perfect representation
0.900 threshold of noticeable loss of quality
0.800 noticeable, but acceptable quality reduction
0.500 barely acceptable quality
0.300 severely degraded quality
0.000 completely degraded quality
Note that most meaningful values in this table are close to 1.
This is due to the fact that quality factors are generally combined
by multiplying them, not by adding them.
When assigning source-quality values, content providers MUST NOT
account for the size of the variant and its impact on transmission
and rendering delays. Any constant rendering delay for a
particular media type (for example due to the startup time of a
helper application) SHOULD be accounted for by the user agent, when
assigning a quality factor to that media type.
5.4 Type, charset, language, and length
The type attribute of a variant description carries the same
information as its Content-Type response header counterpart defined
in [1], except for any charset information, which MUST be carried
in the charset attribute. For, example, the header
Content-Type: text/html; charset=ISO-8859-4
has the counterpart attributes
{type text/html} {charset ISO-8859-4}
The language and length attributes carry the same information as
their Content-* response header counterparts in [1]. The length
attribute, if present, MUST thus reflect the length of the variant
alone, and not the total size of the variant and any objects
inlined or embedded by the variant.
Though all of these attributes are optional, it is often desirable
to include as many attributes as possible, as this will increase
the quality of the negotiation process.
Note: A server is not required to maintain a one-to-one
correspondence between the attributes in the variant description
and the Content-* headers in the variant response. For example,
if the variant description contains a language attribute, the
response does not necessarily have to contain a Content-Language
header. If a Content-Language header is present, it does not
have to contain an exact copy of the information in the language
attribute.
5.5 Features
The features attribute specifies how the presence or absence of
particular feature tags in the user agent affects the overall
quality of the variant. This attribute is covered in section 6.4.
5.6 Description
The description attribute gives a textual description of the
variant. It can be included if the URI and normal attributes of a
variant are considered too opaque to allow interpretation by the
user. If a user agent is showing a menu of available variants
compiled from a variant list, and if a variant has a description
attribute, the user agent SHOULD show the description attribute of
the variant instead of showing the normal attributes of the
variant. The description field uses the UTF-8 character encoding
scheme [7], which is a superset of US-ASCII, with ""%" HEX HEX"
encoding.
5.7 Extension-attribute
The extension-attribute allows future specifications to
incrementally define dimensions of negotiation which cannot be
created by using the feature negotiation framework, and eases
content negotiation experiments. In experimental situations,
servers MUST ONLY generate extension-attributes whose names start
with "x-". User agents SHOULD ignore all extension attributes they
do not recognize. Proxies MUST NOT run a remote variant selection
algorithm if an unknown extension attribute is present in the
variant list.
6 Feature negotiation
This section defines the feature negotiation mechanism. Feature
negotiation has been introduced in section 4.8. Appendix 19
contains examples of feature negotiation.
6.1 Feature tags
A feature tag (ftag) identifies something which can be negotiated
on, for example a property (feature) of a representation, a
capability (feature) of a user agent, or the preference of a user
for a particular type of representation. The use of feature tags
need not be limited to transparent content negotiation, and not
every feature tag needs to be usable in the HTTP transparent
content negotiation framework.
ftag = 1*( ALPHA | DIGIT | "." | "-" )
Feature tags are used in feature sets (section 6.2) and in feature
predicates (section 6.3). Feature predicates are in turn used in
features attributes (section 6.4), which are used in variant
descriptions (section 5). Variant descriptions can be transmitted
in Alternates headers (section 8.3).
Feature tag comparison is case-insensitive. Examples are
tables, fonts, blebber, wolx, screenwidth, colordepth
An example of the use of feature tags in a variant description is:
{"index.html" 1.0 {type text/html} {features tables frames}}
This specification follows general computing practice in that it
places no restrictions on what may be called a feature. Feature
tag definitions SHOULD describe the tag from the viewpoint of the
variant author. For example, a definition could start with `the X
feature tag labels variants which are intended for...'.
At the protocol level, this specification does not distinguish
between different uses of feature tags: a tag will be processed in
the same way, no matter whether it identifies a property,
capability, or preference. For some tags, it may be fluid whether
the tag represents a property, preference, or capability. For
example, in content negotiation on web pages, a "textonly" tag
would identify a capability of a text-only user agent, but the user
of a graphical user agent may use this tag to specify that
text-only content is preferred over graphical content.
While the usage of some tags may be fluid, it is expected that
other tag definitions will strictly limit the usage of a tag to
expressing a property, capability, or preference only. However,
the protocol does not contain any facilities which could enforce
such limitations.
6.1.1 Feature tag values
The definition of a feature tag may state that a feature tag can
have zero, one, or more values associated with it. These values
specialize the meaning of the tag. For example, a feature tag
`paper' could be associated with the values `A4' and `A5'.
tag-value = token | quoted-string
Equality comparison for tag values MUST be done with a
case-sensitive, octet-by-octet comparison, where any ""%" HEX HEX"
encodings MUST be processed as in [1]. A token value XYZ is equal
to a quoted-string value "XYZ".
6.2 Feature sets
The feature set of a user agent is a data structure which records
the capabilities of the user agent and the preferences of the user.
Feature sets are used by local variant selection algorithms (see
appendix 18 for an example). A user agent can use the
Accept-Features header (section 8.2) to make some of the contents
of its feature set known to remote variant selection algorithms.
Structurally, a feature set is a possibly empty set, containing
records of the form
( feature tag , set of feature tag values )
If a record with a feature tag is present in the set, this means
that the user agent implements the corresponding capability, or
that the user has expressed the corresponding preference.
Each record in a feature set has a, possibly empty, set of tag
values. For feature tags which cannot have values associated with
it, this set is always empty. For feature tags which can have
zero, one, or more values associated with it, this set contains
those values currently associated with the tag. If the set of a
featue tag T has the value V in it, it is said that `the tag T is
present with the value V'.
This specification does not define a standard notation for feature
sets. An example of a very small feature set, in a mathematical
notation, is
{ ( "frames" , { } ) ,
( "paper" , { "A4" , "A5" } )
}
As feature registration [4] will be an ongoing process, it is
generally not possible for a user agent to know the meaning of all
feature tags it can possibly encounter in a variant description. A
user agent SHOULD treat all features tags unknown to it as absent
from its feature set.
A user agent may change the contents of its feature set depending
on the type of request, and may also update it to reflect changing
conditions, for example a change in the window size. Therefore,
when considering feature negotiation, one usually talks about `the
feature set of the current request'.
6.3 Feature predicates
Feature predicates are predicates on the contents of feature sets.
They appear in the features attribute of a variant description.
fpred = [ "!" ] ftag
| ftag [ "!" ] "=" tag-value
| ftag "=" "[" numeric-range "]"
numeric-range = [ number ] "-" [ number ]
Feature predicates are used in features attributes (section 6.4),
which are used in variant descriptions (section 5). Variant
descriptions can be transmitted in Alternates headers (section
8.3).
Examples of feature predicates are
blebber, !blebber, paper=a4, colordepth=5, blex!=54,
dpi=[300-599], colordepth=[24-]
Using the feature set of the current request, a user agent SHOULD
compute the truth value of the different feature predicates as
follows.
ftag true if the feature is present, false otherwise
!ftag true if the feature is absent, false otherwise
ftag=V true if the feature is present with the value V,
false otherwise,
ftag!=V true if the feature is not present with the value V,
false otherwise,
ftag=[N-M] true if the feature is present with at least one
numeric value, while the highest value with which it
is present in the range N-M, false otherwise. If N
is missing, the lower bound is 0. If M is missing,
the upper bound is infinity.
As an example, with the feature set
{ ( "blex" , { } ),
( "colordepth" , { "5" } ),
( "UA-media" , { "stationary" } ),
( "paper" , { "A4", "A3" } ) ,
( "x-version" , { "104", "200" } )
}
the following predicates are true:
blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth,
UA-media=stationary, UA-media!=screen, paper=A4, paper =!A0,
colordepth=[ 4 - 6 ], x-version=[100-300], x-version=[200-300]
and the following predicates are false:
!blex, blebber, colordepth=6, colordepth=foo, !colordepth,
screenwidth, screenwidth=640, screenwidth!=640, x-version=99,
UA-media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta
6.4 Features attribute
The features attribute, for which section 5.1 defines the syntax
"{" "features" feature-list "}"
is used in a variant description to specify how the presence or
absence of particular feature tags in the user agent affects the
overall quality of the variant.
feature-list = 1%feature-list-element
feature-list-element = ( fpred | fpred-bag )
[ ":" true-improvement ]
[ "/" false-degradation ]
fpred-bag = "[" 1%fpred "]"
true-improvement = short-float
false-degradation = short-float
Features attributes are used in variant descriptions (section 5).
Variant descriptions can be transmitted in Alternates headers
(section 8.3).
Examples are:
{features !textonly [blebber !wolx] colordepth=3:0.7}
{features !blink/0.5 background:1.5 [blebber !wolx]:1.4/0.8}
The default value for the true-improvement is 1. The default value
for the false-degradation is 0, or 1 if a true-improvement value is
given.
A user agent SHOULD, and a remote variant selection algorithm MUST
compute the quality degradation factor associated with the features
attribute by multiplying all quality degradation factors of the
elements of the feature-list. Note that the result can be a factor
greater than 1.
A feature list element yields its true-improvement factor if the
corresponding feature predicate is true, or if at least one element
of the corresponding fpred-bag is true. The element yields its
false-degradation factor otherwise.
7 Remote variant selection algorithms
A remote variant selection algorithm is a standardized algorithm
by which a server can choose a best variant on behalf of a
negotiating user agent. The use of a remote algorithm can speed up
the negotiation process by eliminating a request-response round
trip.
A remote algorithm typically computes whether the Accept- headers in
the request contain sufficient information to allow a choice, and
if so, which variant is the best variant. This specification does
not define any remote algorithms, but does define a mechanism to
negotiate on the use of such algorithms.
7.1 Version numbers
A version numbering scheme is used to distinguish between different
remote variant selection algorithms.
rvsa-version = major "." minor
major = 1*4DIGIT
minor = 1*4DIGIT
An algorithm with the version number X.Y, with Y>0, MUST be
downwards compatible with all algorithms from X.0 up to X.Y.
Downwards compatibility means that, if supplied with the same
information, the newer algorithm MUST make the same choice, or a
better choice, as the old algorithm. There are no compatibility
requirements between algorithms with different major version
numbers.
8 Content negotiation status codes and headers
This specification adds one new HTTP status code, and introduces
six new HTTP headers. It also extends the semantics of an existing
HTTP/1.1 header.
8.1 506 Variant Also Negotiates
The 506 status code indicates that the server has an internal
configuration error: the chosen variant resource is configured to
engage in transparent content negotiation itself, and is therefore
not a proper end point in the negotiation process.
8.2 Accept-Features
The Accept-Features request header can be used by a user agent to
give information about the presence or absence of certain features
in the feature set of the current request. Servers can use this
information when running a remote variant selection algorithm.
Note: the name `Accept-Features' for this header was chosen
because of symmetry considerations with other Accept- headers,
even though the Accept-Features header will generally not
contain an exhaustive list of features which are somehow
`accepted'. A more accurate name of this header would have been
`Feature-Set-Info'.
Accept-Features = "Accept-Features" ":"
#( feature-expr *( ";" feature-extension ) )
feature-expr = [ "!" ] ftag
| ftag [ "!" ] "=" tag-value
| ftag "=" "{" tag-value "}"
| "*"
feature-extension = token [ "=" ( token | quoted-string ) ]
No feature extensions are defined in this specification. An
example is:
Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
paper = A4, paper!="A2", x-version=104, *
The different feature expressions have the following meaning:
ftag ftag is present
!ftag ftag is absent
ftag=V ftag is present with the value V
ftag!=V ftag is present, but not with the value V
ftag={V} ftag is present with the value V, and not with any
other values
* the expressions in this header do not fully describe
the feature set: feature tags not mentioned in this
header may also be present, and, except for the case
ftag={V}, tags may be present with more values than
mentioned.
Absence of the Accept-Features header in a request is equivalent to
the inclusion of
Accept-Features: *
By using the Accept-Features header, a remote variant selection
algorithm can sometimes determine the truth value of a feature
predicate on behalf of the user agent. For example, with the
header
Accept-Features: blex, !blebber, colordepth={5}, !screenwidth,
paper = A4, paper!="A2", x-version=104, *
the algorithm can determine that the following predicates are true:
blex, colordepth=[4-], colordepth!=6, colordepth, !screenwidth,
paper=A4, colordepth=[4-6]
and that the following predicates are false:
!blex, blebber, colordepth=6, colordepth=foo, !colordepth,
screenwidth, screenwidth=640, screenwidth!=640,
but the truth value of the following predicates cannot be
determined:
UA-media=stationary, UA-media!=screen, paper!=a0,
x-version=[100-300], x-version=[200-300], x-version=99,
UA-media=screen, paper=A0, paper=a4, x-version=[100-199], wuxta
8.3 Alternates
The Alternates response header is used to convey the list of
variants bound to a negotiable resource. This list can also
include directives for any content negotiation process. If a
response from a transparently negotiable resource includes an
Alternates header, this header MUST contain the complete variant
list bound to the negotiable resource. Responses from resources
which do not support transparent content negotiation MAY also use
Alternates headers.
Alternates = "Alternates" ":" variant-list
variant-list = 1#( variant-description
| fallback-variant
| list-directive )
fallback-variant = "{" <"> URI <"> "}"
list-directive = ( "proxy-rvsa" "=" <"> 0#rvsa-version <"> )
| extension-list-directive
extension-list-directive = token [ "=" ( token | quoted-string ) ]
An example is
Alternates: {"paper.html.en" 0.9 {type text/html} {language en}},
{"paper.html.fr" 0.7 {type text/html} {language fr}},
{"paper.ps.en" 1.0 {type application/postscript}
{language en}},
proxy-rvsa="1.0, 2.5"
Any relative URI specified in a variant-description or
fallback-variant field is relative to the request-URI. Only one
fallback-variant field may be present. If the variant selection
algorithm of the user agent finds that all described variants are
unacceptable, then it SHOULD choose the fallback variant, if
present, as the best variant. If the user agent computes the
overall quality values of the described variants, and finds that
several variants share the highest value, then the first variant
with this value in the list SHOULD be chosen as the best variant.
The proxy-rvsa directive restricts the use of remote variant
selection algorithms by proxies. If present, a proxy MUST ONLY use
algorithms which have one of the version numbers listed, or have
the same major version number and a higher minor version number as
one of the versions listed. Any restrictions set by proxy-rvsa
come on top of the restrictions set by the user agent in the
Negotiate request header. The directive proxy-rvsa="" will disable
variant selection by proxies entirely. Clients SHOULD ignore all
extension-list-directives they do not understand.
A variant list may contain multiple differing descriptions of the
same variant. This can be convenient if the variant uses
conditional rendering constructs, or if the variant resource
returns multiple representations using a multipart media type.
8.4 Negotiate
The Negotiate request header can contain directives for any content
negotiation process initiated by the request.
Negotiate = "Negotiate" ":" 1#negotiate-directive
negotiate-directive = "trans"
| "vlist"
| "guess-small"
| rvsa-version
| "*"
| negotiate-extension
negotiate-extension = token [ "=" token ]
Examples are
Negotiate: 1.0, 2.5
Negotiate: *
The negotiate directives have the following meaning
"trans"
The user agent supports transparent content negotiation for
the current request.
"vlist"
The user agent requests that any transparently negotiated
response for the current request includes an Alternates
header with the variant list bound to the negotiable resource.
Implies "trans".
"guess-small"
The user agent allows origin servers to run a custom algorithm
which guesses the best variant for the request, and to return
this variant in a choice response, if the resulting choice
response is smaller than or not much larger than a list
response. The definition of `not much larger' is left to
origin server heuristics. Implies "vlist" and "trans".
rvsa-version
The user agent allows origin servers and proxies to run the
remote variant selection algorithm with the indicated version
number, or with the same major version number and a higher
minor version number. If the algorithm has sufficient
information to choose a best, neighboring variant, the origin
server or proxy MAY return a choice response with this
variant. Implies "trans".
"*"
The user agent allows origin servers and proxies to run any
remote variant selection algorithm. The origin server may
even run algorithms which have not been standardized. If the
algorithm has sufficient information to choose a best,
neighboring variant, the origin server or proxy MAY return a
choice response with this variant. Implies "trans".
Servers SHOULD ignore all negotiate-directives they do not
understand. If the Negotiate header allows a choice between
multiple remote variant selection algorithms which are all
supported by the server, the server SHOULD use some internal
precedence heuristics to select the best algorithm.
8.5 TCN
The TCN response header is used by a server to signal that the
resource is transparently negotiated.
TCN = "TCN" ":" #( response-type
| server-side-override-directive
| tcn-extension )
response-type = "list" | "choice" | "adhoc"
server-side-override-directive = "re-choose" | "keep"
tcn-extension = token [ "=" ( token | quoted-string ) ]
If the resource is not transparently negotiated, a TCN header MUST
NOT be included in any response. If the resource is transparently
negotiated, a TCN header, which includes the response-type value of
the response, MUST be included in every response with a 2xx status
code or any 3xx status code, except 304, in which it MAY be
included. A TCN header MAY also be included, without a
response-type value, in other responses from transparently
negotiated resources.
A server-side override directive MUST be included if the origin
server performed a server-side override when choosing the response.
If the directive is "re-choose", the server MUST include an
Alternates header with the variant bound to the negotiable resource
in the response, and user agent SHOULD use its internal variant
selection algorithm to choose, retrieve, and display the best
variant from this list. If the directive is "keep" the user agent
SHOULD NOT renegotiate on the response, but display it directly, or
act on it directly if it is a redirection response.
Clients SHOULD ignore all tcn-extensions they do not understand.
8.6 Variant-Vary
The Variant-Vary response header can be used in a choice response
to record any vary information which applies to the variant data
(the entity body combined with some of the entity headers)
contained in the response, rather than to the response as a whole.
Variant-Vary = "Variant-Vary" ":" ( "*" | 1#field-name )
Use of the Variant-Vary header is discussed in section 10.2.
9 Cache validators
To allow for correct and efficient caching and revalidation of
negotiated responses, this specification extends the caching model
of HTTP/1.1 [1] in various ways.
This specification does not introduce a `variant-list-max-age'
directive which explicitly bounds the freshness lifetime of a
cached variant list, like the `max-age' Cache-Control directive
bounds the freshness lifetime of a cached response. However, this
specification does ensure that a variant list which is sent at a
time T by the origin server will never be re-used without
revalidation by semantically transparent caches after the time T+M.
This M is the maximum of all freshness lifetimes assigned (using
max-age directives or Expires headers) by the origin server to
a. the responses from the negotiable resource itself, and
b. the responses from its neighboring variant resources
If no freshness lifetimes are assigned by the origin server, M is
the maximum of the freshness lifetimes which were heuristically
assigned by all caches which can re-use the variant list.
9.1 Variant list validators
A variant list validator is an opaque value which acts as the cache
validator of a variant list bound to a negotiable resource.
variant-list-validator = <quoted-string not containing any ";">
If two responses contain the same variant list validator, a cache
can treat the Alternates headers in these responses as equivalent
(though the headers themselves need not be identical).
9.2 Structured entity tags
A structured entity tag consists of a normal entity tag of which
the opaque string is extended with a semicolon followed by the text
(without the surrounding quotes) of a variant list validator:
normal | variant list | structured
entity tag | validator | entity tag
-------------+----------------+-----------------
"etag" | "vlv" | "etag;vlv"
W/"etag" | "vlv" | W/"etag;vlv"
Note that a structured entity tag is itself also an entity tag.
The structured nature of the tag allows caching proxies capable of
transparent content negotiation to perform some optimizations
defined in section 10. When not performing such optimizations, a
structured tag SHOULD be treated as a single opaque value,
according to the general rules in HTTP/1.1. Examples of structured
entity tags are:
"xyzzy;1234" W/"xyzzy;1234" "gonkxxxx;1234" "a;b;c;;1234"
In the last example, the normal entity tag is "a;b;c;" and the
variant list validator is "1234".
If a transparently negotiated response includes an entity tag, it
MUST be a structured entity tag. The variant list validator in the
structured tag MUST act as a validator for the variant list
contained in the Alternates header. The normal entity tag in the
structured tag MUST act as a validator of the entity body in the
response and of all entity headers except Alternates.
9.3 Assigning entity tags to variants
To allow for correct revalidation of transparently negotiated
responses by clients, origin servers SHOULD generate all normal
entity tags for the neighboring variant resources of the negotiable
resource in such a way that
1. the same tag is never used by two different variants,
unless this tag labels exactly the same entity on all
occasions,
2. if one normal tag "X" is a prefix of another normal tag "XY",
then "Y" must never be a semicolon followed by a variant list
validator.
10 Content negotiation responses
If a request on a transparently negotiated resource yields a
response with a 2xx status code or any 3xx status code except 304,
this response MUST always be either a list response, a choice
response, or an adhoc response. These responses MUST always
include a TCN header which specifies their type. Transparently
negotiated responses with other status codes MAY also include a TCN
header.
The conditions under which the different content negotiation
responses may be sent are defined in section 12.1 for origin
servers and in section 13 for proxies.
After having constructed a list, choice, or adhoc response, a
server MAY process any If-No-Match or If-Range headers in the
request message and shorten the response to a 304 (Not Modified) or
206 (Partial Content) response, following the rules in the HTTP/1.1
specification [1]. In this case, the entity tag of the shortened
response will identify it indirectly as a list, choice, or adhoc
response.
10.1 List response
A list response returns the variant list of the negotiable
resource, but no variant data. It can be generated when the server
does not want to, or is not allowed to, return a particular best
variant for the request. If the user agent supports transparent
content negotiation, the list response will cause it to select a
best variant and retrieve it.
A list response MUST contain (besides the normal headers required
by HTTP) a TCN header which specifies the "list" response-type, the
Alternates header bound to the negotiable resource, a Vary header
and (unless it was a HEAD request) an entity body which allows the
user to manually select the best variant.
An example of a list response is
HTTP/1.1 300 Multiple Choices
Date: Tue, 11 Jun 1996 20:02:21 GMT
TCN: list
Alternates: {"paper.html.en" 0.9 {type text/html} {language en}},
{"paper.html.fr" 0.7 {type text/html} {language fr}},
{"paper.ps.en" 1.0 {type application/postscript}
{language en}}
Vary: negotiate, accept, accept-language
ETag: "blah;1234"
Cache-control: max-age=86400
Content-Type: text/html
Content-Length: 227
<h2>Multiple Choices:</h2>
<ul>
<li><a href=paper.1>HTML, English version</a>
<li><a href=paper.2>HTML, French version</a>
<li><a href=paper.3>Postscript, English version</a>
</ul>
Note: A list response can have any status code, but the 300
(Multiple Choices) code is the most appropriate one for HTTP/1.1
clients. Some existing versions of HTTP/1.0 clients are known
to silently ignore 300 responses, instead of handling them
according to the HTTP/1.0 specification [3]. Servers should
therefore be careful in sending 300 responses to non-negotiating
HTTP/1.0 user agents, and in making these responses cacheable.
The 200 (OK) status code can be used instead.
The Vary header in the response SHOULD ensure correct handling by
plain HTTP/1.1 caching proxies. This header can either be
Vary: *
or a more elaborate header; see section 10.6.1.
Only the origin server may construct list responses. Depending on
the status code, a list response is cacheable unless indicated
otherwise.
According to the HTTP/1.1 specification [1], a user agent which
does not support transparent content negotiation will, when
receiving a list response with the 300 status code, display the
entity body included in the response. If the response contains a
Location header, however, the user agent MAY automatically redirect
to this location.
The handling of list responses by clients supporting transparent
content negotiation is described in sections 11.1 and 13.
10.2 Choice response
A choice response returns a representation of the best variant for
the request, and may also return the variant list of the negotiable
resource. It can be generated when the server has sufficient
information to be able to choose the best variant on behalf the
user agent, but may only be generated if this best variant is a
neighboring variant. For request from user agents which do not
support transparent content negotiation, a server may always
generate a choice response, provided that the variant returned is a
neighboring variant. The variant returned in a choice response
need not necessarily be listed in the variant list bound to the
negotiable resource.
A choice response merges a normal HTTP response from the chosen
variant, a TCN header which specifies the "choice" response-type,
and a Content-Location header giving the location of the variant.
Depending on the status code, a choice response is cacheable unless
indicated otherwise.
Origin servers and proxy caches MUST construct choice responses
with the following algorithm (or any other algorithm which gives
equal end results for the client).
In this algorithm, `the current Alternates header' refers to the
Alternates header containing the variant list which was used to
choose the best variant, and `the current variant list validator'
refers to the validator of this list. Section 10.4 specifies how
these two items can be obtained by a proxy cache.
The algorithm consists of four steps.
1. Construct a HTTP request message on the best variant resource
by rewriting the request-URI and Host header (if appropriate)
of the received request message on the negotiable resource.
2. Generate a valid HTTP response message, but not one with the
304 (Not Modified) code, for the request message constructed
in step 1.
In a proxy cache, the response can be obtained from cache
memory, or by passing the constructed HTTP request towards the
origin server. If the request is passed on, the proxy MAY
add, modify, or delete If-None-Match and If-Range headers to
optimize the transaction with the upstream server.
Note: the proxy should be careful not to add entity tags of
non-neighboring variants to If-* (conditional) headers of
the request, as there are no global uniqueness requirements
for these tags.
3. Only in origin servers: check for an origin server
configuration error. If the HTTP response message generated in
step 2 contains a TCN header, then the best variant resource
is not a proper end point in the transparent negotiation
process, and a 506 (Variant Also Negotiates) error response
message SHOULD be generated instead of going to step 4.
4. Add a number of headers to the HTTP response message generated
in step 2.
a. Add a TCN header which specifies the "choice"
response-type.
b. Add a Content-Location header giving the location of the
chosen variant. Delete any Content-Location header which
was already present.
Note: According to the HTTP/1.1 specification [1], if
the Content-Location header contains a relative URI,
this URI is relative to the URI in the Content-Base
header, if present, and relative to the request-URI if
no Content-Base header is present.
c. If any Vary headers are present in the response message
from step 2, add, for every Vary header, a Variant-Vary
header with a copy of the contents of this Vary header.
d. Delete any Alternates headers which are present in in the
response. Now, the current Alternates header MUST be added
if this is required by the Negotiate request header, or if
the server returns "re-choose" in the TCN response header.
Otherwise, the current Alternates header MAY be added.
Note: It is usually a good strategy to always add the
current Alternates header, unless it is very large
compared to the rest of the response.
e. Add a Vary header to ensure correct handling by plain
HTTP/1.1 caching proxies. This header can either be
Vary: *
or a more elaborate header, see section 10.6.
f. To ensure compatibility with HTTP/1.0 caching proxies which
do not recognize the Vary header, an Expires header with a
date in the past MAY be added. See section 10.7 for more
information.
g. If an ETag header is present in the response message from
step 2, then extend the entity tag in that header with the
current variant list validator, as specified in section
9.2.
Note: Step g. is required even if the variant list
itself is not added in step d.
h. Only in proxy caches: set the Age header of the response to
max( variant_age , alternates_age )
where variant_age is the age of the variant response
obtained in step 2, calculated according to the rules in
the HTTP/1.1 specification [1], and alternates_age is the
age of the Alternates header added in step d, calculated
according to the rules in section 10.4.
Note that a server can shorten the response produced by the above
algorithm to a 304 (Not Modified) response if an If-None-Match
header in the original request allows it. If this is the case, an
implementation of the above algorithm can avoid the unnecessary
internal construction of full response message in step 2, it need
only construct the parts which end up in the final 304 response. A
proxy cache which implements this optimization can sometimes
generate a legal 304 response even if it has not cached the variant
data itself.
An example of a choice response is:
HTTP/1.1 200 OK
Date: Tue, 11 Jun 1996 20:05:31 GMT
TCN: choice
Content-Type: text/html
Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
Content-Length: 5327
Cache-control: max-age=604800
Content-Location: paper.1
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Etag: "gonkyyyy;1234"
Vary: negotiate, accept, accept-language
Expires: Thu, 01 Jan 1980 00:00:00 GMT
<title>A paper about ....
10.3 Adhoc response
An adhoc response can be sent by an origin server as an extreme
measure, to achieve compatibility with a non-negotiating or buggy
client if this compatibility cannot be achieved by sending a list
or choice response. There are very little requirements on the
contents of an adhoc response. An adhoc response MUST have a TCN
header which specifies the "adhoc" response-type, and a Vary header
if the response is cacheable. It MAY contain the Alternates header
bound to the negotiable resource.
Any Vary header in the response SHOULD ensure correct handling by
plain HTTP/1.1 caching proxies. This header can either be
Vary: *
or a more elaborate header, see section 10.6.1. Depending on the
status code, an adhoc response is cacheable unless indicated
otherwise.
As an example of the use of an adhoc response, suppose that the
variant resource "redirect-to-blah" yields redirection (302)
responses. A choice response with this variant could look as
follows:
HTTP/1.1 302 Moved Temporarily
Date: Tue, 11 Jun 1996 20:02:28 GMT
TCN: choice
Content-location: redirect-to-blah
Location: http://blah.org/
Content-Type: text/html
Content-Length: 62
This document is available <a href=http://blah.org/>here</a>.
Suppose that the server knows that the receiving user agent has a
bug, which causes it to crash on responses which contain both a
Content-Location and a Location header. The server could then work
around this bug by performing a server-side override and sending
the following adhoc response instead:
HTTP/1.1 302 Moved Temporarily
Date: Tue, 11 Jun 1996 20:02:28 GMT
TCN: adhoc, keep
Location: http://blah.org/
Content-Type: text/html
Content-Length: 62
This document is available <a href=http://blah.org/>here</a>.
10.4 Reusing the Alternates header
If a proxy cache has available a negotiated response which is
cacheable, fresh, and has ETag and Alternates headers, then it MAY
extract the Alternates header and associated variant list validator
from the response, and reuse them (without unnecessary delay) to
negotiate on behalf of the user agent (section 13) or to construct
a choice response (section 10.2). The age of the extracted
Alternates header is the age of the response from which it is
extracted, calculated according to the rules in the HTTP/1.1
specification [1].
10.5 Extracting a normal response from a choice response
If a proxy receives a choice response, it MAY extract and cache the
normal HTTP response contained therein. The normal response can be
extracted by taking a copy of the choice response and then deleting
any Content-Location, Alternates, and Vary headers, renaming any
Variant-Vary headers to Vary headers, and shortening the structured
entity tag in any ETag header to a normal entity tag.
This normal response MAY be cached (as a HTTP response to the
variant request as constructed in step 1. of section 10.2) and
reused to answer future direct requests on the variant resource,
according to the rules in the HTTP/1.1 specification [1].
Note: The caching of extracted responses can decrease the
upstream bandwidth usage with up to a factor 2, because two
independent HTTP/1.1 cache entries, one associated with the
negotiable resource URI and one with the variant URI, are
created in the same transaction. Without this optimization,
both HTTP/1.1 cache entries can only be created by transmitting
the variant data twice.
For security reasons (see section 14.2), an extracted normal
response MUST NEVER be cached if belongs to a non-neighboring
variant resource. If the choice response claims to contain data
for a non-neighboring variant resource, the proxy SHOULD reject the
choice response as a probable spoofing attempt.
10.6 Elaborate Vary headers
If a HTTP/1.1 [1] server can generate varying responses for a
request on some resource, then the server MUST include a Vary
header in these responses if they are cacheable. This Vary header
is a signal to HTTP/1.1 caches that something special is going on.
It prevents the caches from returning the currently chosen response
for every future request on the resource.
Servers engaging in transparent content negotiation will generate
varying responses. Therefore, cacheable list, choice, and adhoc
responses MUST always include a Vary header.
The most simple Vary header which can be included is
Vary: *
This header leaves the way in which the response is selected by the
server completely unspecified.
A more elaborate Vary header MAY be used to allow for certain
optimizations in HTTP/1.1 caches which do not have specific
optimizations for transparent content negotiation, but which do
cache multiple variant responses for one resource. Such a more
elaborate Vary header lists all request headers which can be used
by the server when selecting a response for a request on the
resource.
10.6.1 Construction of an elaborate Vary header
Origin servers can construct a more elaborate Vary header in the
following way. First, start with the header
Vary: negotiate
`negotiate' is always included because servers use the information
in the Negotiate header when choosing between a list, choice, or
adhoc response.
Then, if any of the following attributes is present in any variant
description in the Alternates header, add the corresponding header
name to the Vary header
attribute | header name to add
-----------+---------------------
type | accept
charset | accept-charset
language | accept-language
features | accept-features
The Vary header constructed in this way specifies the response
variation which can be caused by the use of a variant selection
algorithm in proxies. If the origin server will in some cases, for
example if contacted by a non-negotiating user agent, use a custom
negotiation algorithm which takes additional headers into account,
these names of these headers SHOULD also be added to the Vary
header.
10.6.2 Caching of an elaborate Vary header
A proxy cache cannot construct an elaborate vary header using the
method above, because this method requires exact knowledge of any
custom algorithms present in the origin server. However, when
extracting an Alternates header from a response (section 10.4)
caches MAY also extract the Vary header in the response, and reuse
it along with the Alternates header. A clean Vary header can
however only be extracted if the variant does not vary itself,
i.e. if a Variant-Vary header is absent.
10.7 Adding an Expires header to ensure HTTP/1.0 compatibility
To ensure compatibility with HTTP/1.0 caching proxies which do not
recognize the Vary header, an Expires header with a date in the
past can be added to the response, for example
Expires: Thu, 01 Jan 1980 00:00:00 GMT
If this is done by an origin server, the server SHOULD usually also
include a Cache-Control header for the benefit of HTTP/1.1 caches,
for example
Cache-Control: max-age=604800
which overrides the freshness lifetime of zero seconds specified by
the included Expires header.
Note: This specification only claims downwards compatibility
with the HTTP/1.0 proxy caches which implement the HTTP/1.0
specification [3]. Some legacy proxy caches which return the
HTTP/1.0 protocol version number do not honor the HTTP/1.0
Expires header as specified in [3]. Methods for achieving
compatibility with such proxy caches are beyond the scope of
this specification.
10.8 Negotiation on content encoding
Negotiation on the content encoding of a response is orthogonal to
transparent content negotiation. The rules for when a content
encoding may be applied are the same as in HTTP/1.1: servers MAY
content-encode responses that are the result of transparent content
negotiation whenever an Accept-Encoding header in the request
allows it. When negotiating on the content encoding of a cacheable
response, servers MUST add the accept-encoding header name to the
Vary header of the response, or add `Vary: *'.
Servers SHOULD always be able to provide unencoded versions of
every transparently negotiated response. This means in particular
that every variant in the variant list SHOULD at least be available
in an unencoded form.
Like HTTP/1.1, this specification allows proxies to encode or
decode relayed or cached responses on the fly, unless explicitly
forbidden by a Cache-Control directive. The encoded or decoded
response still contains the same variant as far as transparent
content negotiation is concerned. Note that HTTP/1.1 requires
proxies to add a Warning header if the encoding of a response is
changed.
11 User agent support for transparent negotiation
This section specifies the requirements a user agent needs to
satisfy in order to support transparent negotiation. If the user
agent contains an internal cache, this cache MUST conform to the
rules for proxy caches in section 13.
11.1 Handling of responses
If a list response is received when a resource is accessed, the
user agent MUST be able to automatically choose, retrieve, and
display the best variant, or display an error message if none of
the variants are acceptable.
If a choice response is received when a resource is accessed, the
usual action is to automatically display the enclosed entity.
However, if a remote variant selection algorithm which was enabled
could have made a choice different from the choice the local
algorithm would make, the user agent MAY apply its local algorithm
to any variant list in the response, and automatically retrieve and
display another variant if the local algorithm makes an other
choice.
When receiving a choice response, a user agent SHOULD check if
variant resource is a neighboring variant resource of the
negotiable resource. If this is not the case, the user agent
SHOULD reject the choice response as a probable spoofing attempt
and display an error message, for example by internally replacing
the choice response with a 502 (bad gateway) response.
11.2 Presentation of a transparently negotiated resource
If the user agent is displaying a variant which is not an embedded
or inlined object and which is the result of transparent content
negotiation, the following requirements apply.
1. The user agent SHOULD allow the user to review a list of all
variants bound to the negotiable resource, and to manually
retrieve another variant if desired. There are two general
ways of providing such a list. First, the information in the
Alternates header of the negotiable resource could be used to
make an annotated menu of variants. Second, the entity
included in a list response of the negotiable resource could be
displayed. Note that a list response can be obtained by doing
a GET request which only has the "trans" directive in the
Negotiate header.
2. The user agent SHOULD make available though its user interface
some indication that the resource being displayed is a
negotiated resource instead of a plain resource. It SHOULD
also allow the user to examine the variant list included in the
Alternates header. Such a notification and review mechanism is
needed because of privacy considerations, see section 14.1.
3. If the user agent shows the URI of the displayed information to
the user, it SHOULD be the negotiable resource URI, not the
variant URI that is shown. This encourages third parties, who
want to refer to the displayed information in their own
documents, to make a hyperlink to the negotiable resource as a
whole, rather than to the variant resource which happens to be
shown. Such correct linking is vital for the interoperability
of content across sites. The user agent SHOULD however also
provide a means for reviewing the URI of the particular variant
which is currently being displayed.
4. Similarly, if the user agent stores a reference to the
displayed information for future use, for example in a hotlist,
it SHOULD store the negotiable resource URI, not the
variant URI.
It is encouraged, but not required, that some of the above
functionality is also made available for inlined or embedded
objects, and when a variant which was selected manually is being
displayed.
12 Origin server support for transparent negotiation
12.1 Requirements
To implement transparent negotiation on a resource, the origin
server MUST be able to send a list response when getting a GET
request on the resource. It SHOULD also be able to send
appropriate list responses for HEAD requests. When getting a
request on a transparently negotiable resource, the origin server
MUST NEVER return a response with a 2xx status code or any 3xx
status code, except 304, which is not a list, choice, or adhoc
response.
If the request includes a Negotiate header with a "vlist" or
"trans" directive, but without any directive which allows the
server to select a best variant, a list response MUST ALWAYS be
sent, except when the server is performing a server-side override
for bug compatibility. If the request includes a Negotiate header
with a "vlist" or "guess-small" directive, an Alternates header
with the variant list bound to the negotiable resource MUST ALWAYS
be sent in any list, choice, or adhoc response, except when the
server is performing a server-side override for bug compatibility.
If the Negotiate header allows it, the origin server MAY run a
remote variant selection algorithm. If the algorithm has
sufficient information to choose a best variant, and if the best
variant is a neighboring variant, the origin server MAY return a
choice response with this variant.
When getting a request on a transparently negotiable resource from
a user agent which does not support transparent content
negotiation, the origin server MAY use a custom algorithm to select
between sending a list, choice, or adhoc response.
The following table summarizes the rules above.
|Req on |Usr agnt|server- | Response may be: |
|trans neg|capable |side +------+------+------+------+------+
|resource?|of TCN? |override?|list |choice|adhoc |normal|error |
+---------+--------+---------+------+------+------+------+------+
| Yes | Yes | No |always|smt(*)|never |never |always|
| Yes | Yes | Yes |always|always|always|never |always|
+---------+--------+---------+------+------+------+------+------+
| Yes | No | - |always|always|always|never |always|
+---------+--------+---------+------+------+------+------+------+
| No | - | - |never |never |never |always|always|
+---------+--------+---------+------+------+------+------+------+
(*) sometimes, when allowed by the Negotiate request header
Negotiability is a binary property: a resource is either
transparently negotiated, or it is not. Origin servers SHOULD NOT
vary the negotiability of a resource, or the variant list bound to
that resource, based on the request headers which are received.
The variant list and the property of being negotiated MAY however
change through time. The Cache-Control header can be used to
control the propagation of such time-dependent changes through
caches.
It is the responsibility of the author of the negotiable resource
to ensure that all resources in the variant list serve the intended
content, and that the variant resources do not engage in
transparent content negotiation themselves.
12.2 Negotiation on transactions other than GET and HEAD
If a resource is transparently negotiable, this only has an impact
on the GET and HEAD transactions on the resource. It is not
possible (under this specification) to do transparent content
negotiation on the direct result of a POST request.
However, a POST request can return an unnegotiated 303 (See Other)
response which causes the user agent to do a GET request on a
second resource. This second resource could then use transparent
content negotiation to return an appropriate final response. The
figure below illustrates this.
Server ______ proxy ______ proxy ______ user
x.org cache cache agent
< -------------------------------------
| POST http://x.org/cgi/submit
| <form contents in request body>
|
-------------------------------------- >
303 See Other |
Location: http://x.org/result/OK |
|
< -------------------------------------
| GET http://x.org/result/OK
| small Accept- headers
|
able to choose on
behalf of user agent
|
------------------------------------- >
choice response with |
..result/OK.nl variant |
displays OK.nl
See the HTTP/1.1 specification [1] for details on the 303 (See
Other) status code. Note that this status code is not understood
by some HTTP/1.0 clients.
13 Proxy support for transparent negotiation
Transparent content negotiation is an extension on top of HTTP/1.x.
It is designed to work through any proxy which only implements the
HTTP/1.1 specification [1]. If Expires headers are added as
discussed in section 10.7, negotiation will also work though
proxies which implement HTTP/1.0 [3]. Thus, every HTTP/1.0 or
HTTP/1.1 proxy provides support for transparent content
negotiation. However, if it is to be claimed that a HTTP/1.x proxy
offers transparent content negotiation services, at least one of
the specific optimizations below MUST be implemented.
An HTTP/1.x proxy MUST ONLY optimize (change) the HTTP traffic
flowing through it in ways which are explicitly allowed by the
specification(s) it conforms to. A proxy which supports
transparent content negotiation on top of HTTP/1.x MAY perform the
optimizations allowed for by HTTP/1.x. In addition, it MAY perform
three additional optimizations, defined below, on the HTTP traffic
for transparently negotiated resources and their neighboring
variant resources.
First, when getting a request on a transparently negotiable
resource from a user agent which supports transparent content
negotiation, the proxy MAY return any cached, fresh list response
from that resource, even if the selecting request headers, as
specified by the Vary header, do not match.
Second, when allowed by the user agent and origin server, a proxy
MAY reuse an Alternates header taken from a previous response
(section 10.4) to run a remote variant selection algorithm. If the
algorithm has sufficient information to choose a best variant, and
if the best variant is a neighboring variant, the proxy MAY return
a choice response with this variant.
Third, if a proxy receives a choice response, it MAY extract and
cache the normal response embedded therein, as described in section
10.5.
14 Security and privacy considerations
14.1 Accept- headers revealing information of a private nature
Accept- headers, in particular Accept-Language headers, may reveal
information which the user would rather keep private unless it will
directly improve the quality of service. For example, a user may
not want to send language preferences to sites which do not offer
multi-lingual content. The transparent content negotiation
mechanism allows user agents to omit sending of the Accept-Language
header by default, without adversely affecting the outcome of the
negotiation process if transparently negotiated multi-lingual
content is accessed.
However, even if Accept- headers are never sent, the automatic
selection and retrieval of a variant by a user agent will reveal a
preference for this variant to the server. A malicious service
author could provide a page with `fake' negotiability on
(ethnicity-correlated) languages, with all variants actually being
the same English document, as a means of obtaining
privacy-sensitive information. Such a plot would however be
visible to an alert victim if the list of available variants and
their properties is reviewed.
Some additional privacy considerations connected to Accept- headers
are discussed in [1].
14.2 Spoofing of responses from variant resources
The caching optimization in section 10.5 gives the implementer of a
negotiable resource control over the responses cached for all
neighboring variant resources. This is a security problem if a
neighboring variant resource belongs to another author. To provide
security in this case, the HTTP server will have to filter the
Content-Location headers in the choice responses generated by the
negotiable resource implementation.
14.3 Security holes revealed by negotiation
Malicious servers could use transparent content negotiation as a
means of obtaining information about security holes which may be
present in user agents. This is a risk in particular for
negotiation on the availability of scripting languages and
libraries.
15 Acknowledgments
Work on HTTP content negotiation has been done since at least 1993.
The authors are unable to trace the origin of many of the ideas
incorporated in this document. This specification builds on an
earlier incomplete specification of content negotiation recorded in
[2]. Many members of the HTTP working group have contributed to
the negotiation model in this specification. The authors wish to
thank the individuals who have commented on earlier versions of
this document, including Brian Behlendorf, Daniel DuBois, Martin
J. Duerst, Roy T. Fielding, Jim Gettys, Yaron Goland, Dirk van
Gulik, Ted Hardie, Graham Klyne, Scott Lawrence, Larry Masinter,
Jeffrey Mogul, Henrik Frystyk Nielsen, Frederick G.M. Roeber, Paul
Sutton, and Klaus Weide.
16 References
[1] R. Fielding, J. Gettys, J. C. Mogul, H. Frystyk, and
T. Berners-Lee. Hypertext Transfer Protocol -- HTTP/1.1. RFC
2068, HTTP Working Group, January, 1997.
[2] Roy T. Fielding, Henrik Frystyk Nielsen, and Tim Berners-Lee.
Hypertext Transfer Protocol -- HTTP/1.1. Internet-Draft
draft-ietf-http-v11-spec-01.txt, HTTP Working Group, January,
1996.
[3] T. Berners-Lee, R. Fielding, and H. Frystyk. Hypertext
Transfer Protocol -- HTTP/1.0. RFC 1945. MIT/LCS, UC Irvine,
May 1996.
[4] K. Holtman, A. Mutz. Feature Tag Registration Procedures.
Internet-Draft draft-ietf-http-feature-reg-01.txt, HTTP Working
Group, July 7, 1997.
[5] K. Holtman, A. Mutz. HTTP Remote Variant Selection Algorithm
-- RVSA/1.0. Internet-Draft draft-ietf-http-rvsa-v10-01.txt,
HTTP Working Group. March 23, 1997.
[6] S. Bradner. Key words for use in RFCs to Indicate Requirement
Levels. RFC 2119. Harvard University, March 1997.
[7] F. Yergeau. UTF-8, a transformation format of Unicode and ISO
10646. RFC 2044. Alis Technologies, October 1996.
17 Authors' addresses
Koen Holtman
Technische Universiteit Eindhoven
Postbus 513
Kamer HG 6.57
5600 MB Eindhoven (The Netherlands)
Email: koen@win.tue.nl
Andrew H. Mutz
Hewlett-Packard Company
1501 Page Mill Road 3U-3
Palo Alto CA 94304, USA
Fax +1 415 857 4691
Email: mutz@hpl.hp.com
18 Appendix: Example of a local variant selection algorithm
A negotiating user agent will choose the best variant from a
variant list with a local variant selection algorithm. This
section contains an example of such an algorithm.
The inputs of the algorithm are a variant list from an Alternates
header, and an agent-side configuration database, which contains
- the feature set of the current request,
- a collection of quality values assigned to media types,
languages, and charsets for the current request, following the
model of the corresponding HTTP/1.1 [1] Accept* headers,
- a table which lists `forbidden' combinations of media types and
charsets, i.e. combinations which cannot be displayed because
of some internal user agent limitation.
The output of the algorithm is either the best variant, or the
conclusion that none of the variants are acceptable.
18.1 Computing overall quality values
As a first step in the local variant selection algorithm, the
overall qualities associated with all variant descriptions in the
list are computed.
The overall quality Q of a variant description is the value
Q = round5( qs * qt * qc * ql * qf * qa )
where rounds5 is a function which rounds a floating point value to
5 decimal places after the point. It is assumed that the user
agent can run on multiple platforms: the rounding function makes
the algorithm independent of the exact characteristics of the
underlying floating point hardware.
The factors qs, qt, qc, ql, qf, and qa are determined as follows.
qs Is the source quality factor in the variant description.
qt The media type quality factor is 1 if there is no type
attribute in the variant description. Otherwise, it is the
quality value assigned to this type by the configuration
database. If the database does not assign a value, then the
factor is 0.
qc The charset quality factor is 1 if there is no charset
attribute in the variant description. Otherwise, it is the
quality value assigned to this charset by the configuration
database. If the database does not assign a value, then the
factor is 0.
ql The language quality factor is 1 if there is no language
attribute in the variant description. Otherwise, it is the
highest quality value the configuration database assigns to
any of the languages listed in the language attribute. If
the database does not assign a value to any of the languages
listed, then the factor is 0.
qf The features quality factor is 1 if there is no features
attribute in the variant description. Otherwise, it is the
quality degradation factor computed for the features
attribute using the feature set of the current request.
qa The quality adjustment factor is 0 if the variant description
lists a media type - charset combination which is `forbidden'
by the table, and 1 otherwise.
As an example, if a variant list contains the variant description
{"paper.2" 0.7 {type text/html} {language fr}}
and if the configuration database contains the quality value
assignments
types: text/html;q=1.0, type application/postscript;q=0.8
languages: en;q=1.0, fr;q=0.5
then the local variant selection algorithm will compute the overall
quality for the variant description as follows:
{"paper.2" 0.7 {type text/html} {language fr}}
| | |
| | |
V V V
round5 ( 0.7 * 1.0 * 0.5 ) = 0.35000
With same configuration database, the variant list
{"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript} {language en}}
would yield the following computations:
round5 ( qs * qt * qc * ql * qf * qa ) = Q
--- --- --- --- --- ---
paper.1: 0.9 * 1.0 * 1.0 * 1.0 * 1.0 * 1.0 = 0.90000
paper.1: 0.7 * 1.0 * 1.0 * 0.5 * 1.0 * 1.0 = 0.35000
paper.3: 1.0 * 0.8 * 1.0 * 1.0 * 1.0 * 1.0 = 0.80000
18.2 Determining the result
Using all computed overall quality values, the end result of the
local variant selection algorithm is determined as follows.
If all overall quality values are 0, then the best variant is
the fallback variant, if there is one in the list, else the
result is the conclusion that none of the variants are
acceptable.
If at least one overall quality value is greater than 0, then the
best variant is the variant which has the description with the
highest overall quality value, or, if there are multiple variant
descriptions which share the highest overall quality value, the
variant of the first variant description in the list which has this
highest overall quality value.
18.3 Ranking dimensions
Consider the following variant list:
{"paper.greek" 1.0 {language el} {charset ISO-8859-7}},
{"paper.english" 1.0 {language en} {charset ISO-8859-1}}
It could be the case that the user prefers the language "el" over
"en", while the user agent can render "ISO-8859-1" better than
"ISO-8859-7". The result is that in the language dimension, the
first variant is best, while the second variant is best in the
charset dimension. In this situation, it would be preferable to
choose the first variant as the best variant: the user settings in
the language dimension should take precedence over the hard-coded
values in the charset dimension.
To express this ranking between dimensions, the user agent
configuration database should have a higher spread in the quality
values for the language dimension than for the charset dimension.
For example, with
languages: el;q=1.0, en-gb;q=0.7, en;q=0.6, da;q=0, ...
charsets: ISO-8859-1;q=1.0, ISO-8859-7;q=0.95,
ISO-8859-5;q=0.97, unicode-1-1;q=0, ...
the first variant will have an overall quality of 0.95000, while
the second variant will have an overall quality 0.70000. This
makes the first variant the best variant.
19 Appendix: feature negotiation examples
This appendix contains examples of the use of feature tags in
variant descriptions. The tag names used here are examples only,
they do not in general reflect the tag naming scheme proposed in
[4].
19.1 Use of feature tags
Feature tags can be used in variant lists to express the quality
degradation associated with the presence or absence of certain
features. One example is
{"index.html.plain" 0.7 },
{"index.html" 1.0 {features tables frames}}
Here, the "{features tables frames}" part expresses that index.html
uses the features tagged as tables and frames. If these features
are absent, the overall quality of index.html degrades to 0.
Another example is
{"home.graphics" 1.0 {features !textonly}},
{"home.textonly" 0.7 }
where the "{features !textonly}" part expresses that home.graphics
requires the absence of the textonly feature. If the feature is
present, the overall quality of home.graphics degrades to 0.
The absence of a feature need not always degrade the overall quality
to 0. In the example
{"x.html.1" 1.0 {features fonts/0.7}}
the absence of the fonts feature degrades the quality with a factor
of 0.7. "fonts/0.7" can be pronounced as "fonts, or a degradation
of 0.7". Finally, in the example
{"y.html" 1.0 {features [blebber wolx] }}
The "[blebber wolx]" expresses that y.html requires the presence of
the blebber feature or the wolx feature. This construct can be
used in a number of cases:
1. blebber and wolx actually tag the same feature, but they were
registered by different people, and some user agents say they
support blebber while others say they support wolx.
2. blebber and wolx are HTML tags of different vendors which
implement the same functionality, and which are used
together in y.html without interference.
3. blebber and wolx are HTML tags of different vendors which
implement the same functionality, and y.html uses the tags in
a conditional HTML construct.
4. blebber is a complicated HTML tag with only a sketchy
definition, implemented by one user agent vendor, and wolx
indicates implementation of a well-defined subset of the
blebber tag by some other vendor(s). y.html uses only this
well-defined subset.
19.2 Use of numeric feature tags
As an example of negotiation in a numeric area, the following
variant list describes four variants with title graphics designed
for increasing screen widths:
{"home.pda" 1.0 {features screenwidth=[-199] }},
{"home.narrow" 1.0 {features screenwidth=[200-599] }},
{"home.normal" 1.0 {features screenwidth=[600-999] }},
{"home.wide" 1.0 {features screenwidth=[1000-] }},
{"home.normal"}
The last element of the list specifies a safe default for user
agents which do not implement screen width negotiation. Such user
agents will reject the first four variants as unusable, as they
seem to rely on a feature which they do not understand.
19.3 Feature tag design
When designing a new feature tag, it is important to take into
account that existing user agents, which do not recognize the new
tag will treat the feature as absent. In general, a new feature
tag needs to be designed in such a way that absence of the tag is
the default case which reflects current practice. If this design
principle is ignored, the resulting feature tag will generally be
unusable.
As an example, one could try to support negotiation between
monochrome and color content by introducing a `color' feature tag,
the presence of which would indicate the capability to display
color graphics. However, if this new tag is used in a variant
list, for example
{"rainbow.gif" 1.0 {features color} }
{"rainbow.mono.gif" 0.6 {features !color}}
then existing user agents, which would not recognize the color tag,
would all display the monochrome rainbow. The color tag is
therefore unusable in situations where optimal results for existing
user agents are desired. To provide for negotiation in this area,
one must introduce a `monochrome' feature tag; its presence
indicates that the user agent can only render (or the user prefers
to view) monochrome graphics.
20 Appendix: origin server implementation considerations
20.1 Implementation with a CGI script
Transparent content negotiation has been designed to allow a broad
range of implementation options at the origin server side. A very
minimal implementation can be done using the CGI interface. The
CGI script below is an example.
#!/bin/sh
cat - <<'blex'
TCN: list
Alternates: {"stats.tables.html" 1.0 {type text/html} {features
tables}}, {"stats.html" 0.8 {type text/html}}, {"stats.ps" 0.95
{type application/postscript}}
Vary: *
Content-Type: text/html
<title>Multiple Choices for Web Statistics</title>
<h2>Multiple Choices for Web Statistics:</h2>
<ul>
<li><a href=stats.tables.html>Version with HTML tables</a>
<p>
<li><a href=stats.html>Version without HTML tables</a>
<p>
<li><a href=stats.ps>Postscript version</a>
</ul>
blex
The Alternates header in the above script must be read as a single
line. The script always generates a list response with the 200
(OK) code, which ensures compatibility with non-negotiating
HTTP/1.0 agents.
20.2 Direct support by HTTP servers
Sophisticated HTTP servers could make a transparent negotiation
module available to content authors. Such a module could
incorporate a remote variant selection algorithm and an
implementation of the algorithm for generating choice responses
(section 10.2). The definition of interfaces to such modules is
beyond the scope of this specification.
20.3 Web publishing tools
Web publishing tools could automatically generate several variants
of a document (for example the original TeX version, a HTML version
with tables, a HTML version without tables, and a Postscript
version), together with an appropriate variant list in the
interface format of a HTTP server transparent negotiation module.
This would allow documents to be published as transparently
negotiable resources.
21 Appendix: Example of choice response construction
The following is an example of the construction of a choice
response by a proxy cache which supports HTTP/1.1 and transparent
content negotiation. The use of the HTTP/1.1 conditional request
mechanisms is also shown.
Assume that a user agent has cached a variant list with the
validator "1234" for the negotiable resource http://x.org/paper.
Also assume that it has cached responses from two neighboring
variants, with the entity tags "gonkyyyy" and W/"a;b". Assume that
all three user agent cache entries are stale: they would need to be
revalidated before the user agent can use them. If
http://x.org/paper accessed in this situation, the user agent could
send the following request to its proxy cache:
GET /paper HTTP/1.1
Host: x.org
User-Agent: WuxtaWeb/2.4
Negotiate: 1.0
Accept: text/html, application/postscript;q=0.4, */*
Accept-Language: en
If-None-Match: "gonkyyyy;1234", W/"a;b;1234"
Assume that the proxy cache has cached the same three items as the
user agent, but that it has revalidated the variant list 8000
seconds ago, so that the list is still fresh for the proxy. This
means that the proxy can run a remote variant selection algorithm
on the list and the incoming request.
Assume that the remote algorithm is able to choose paper.en.html as
the best variant. The proxy can now construct a choice response,
using the algorithm in section 10.2. In steps 1 and 2 of the
algorithm, the proxy can construct the following conditional
request on the best variant, and send it to the origin server:
GET /paper.html.en HTTP/1.1
Host: x.org
User-Agent: WuxtaWeb/2.4
Negotiate: 1.0
Accept: text/html, application/postscript;q=0.4, */*
Accept-Language: en
If-None-Match: "gonkyyyy", W/"a;b"
Via: 1.1 fred
On receipt of the response
HTTP/1.1 304 Not Modified
Date: Tue, 11 Jun 1996 20:05:31 GMT
Etag: "gonkyyyy"
from the origin server, the proxy can use its freshly revalidated
paper.html.en cache entry to expand the response to a non-304
response:
HTTP/1.1 200 OK
Date: Tue, 11 Jun 1996 20:05:31 GMT
Content-Type: text/html
Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
Content-Length: 5327
Cache-control: max-age=604800
Etag: "gonkyyyy"
Via: 1.1 fred
Age: 0
<title>A paper about ....
Using this 200 response, the proxy can construct a choice response
in step 4 of the algorithm:
HTTP/1.1 200 OK
Date: Tue, 11 Jun 1996 20:05:31 GMT
TCN: choice
Content-Type: text/html
Last-Modified: Mon, 10 Jun 1996 10:01:14 GMT
Content-Length: 5327
Cache-control: max-age=604800
Content-Location: paper.html.en
Alternates: {"paper.html.en" 0.9 {type text/html} {language en}},
{"paper.html.fr" 0.7 {type text/html} {language fr}},
{"paper.ps.en" 1.0 {type application/postscript}
{language en}}
Etag: "gonkyyyy;1234"
Vary: negotiate, accept, accept-language
Expires: Thu, 01 Jan 1980 00:00:00 GMT
Via: 1.1 fred
Age: 8000
<title>A paper about ....
The choice response can subsequently be shortened to a 304
response, because of the If-None-Match header in the original
request from the user agent. Thus, the proxy can finally return
HTTP/1.1 304 Not Modified
Date: Tue, 11 Jun 1996 20:05:31 GMT
Etag: "gonkyyyy;1234"
Content-Location: paper.html.en
Vary: negotiate, accept, accept-language
Expires: Thu, 01 Jan 1980 00:00:00 GMT
Via: 1.1 fred
Age: 8000
to the user agent.
Expires: January 26, 1998